The head bothers me a little as well. It's a small head already, and when you beef up the body with extra parts then it looks even smaller.
I was holding out with the Prime kit, and then I saw them on sale at K Mart this week for $22 so I bought two of them and used the extra parts from the second kit. I made some minor changes since this pic, but this is still how it looks.
It seems like "doubling up" on the kits works better than just one... I did the same to Bumblebee awhile ago:

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